84 THE CHICAGO ACADEMY OF SCIENCES. 



It would appear that since early Eocene Time this plateau region, 

 known later as the Great Basin, had been the scene of some of the 

 most stupendous physical changes in the history of the earth. At the 

 close of the Mesozoic Era this region was notably subjected to warp- 

 ings and faultings which elevated much of the land, producing irregular 

 mountainous tracts capable of rapid erosion. Basins were formed here 

 and there in which lakes of large size developed. 1 It seems not unlikely 

 that from this time (the Lower Eocene Period) to the present there 

 existed in the area of the Great Basin a continuous series of lakes, 

 large and small, of which the Pleistocene lakes Lahontan and Bonne- 

 ville were the last, and of which Great Salt Lake is a diminishing 

 descendant. 



That the fresh-water faunas contemporaneous with the Lymnaeas 

 were relatively large and varied in character is attested by the presence 

 of the genera Unio, Auricula, Tortacella, Planorbis, Physa, Neritina, 

 Pachymelania, Pyrgulifera, Hydrobia, Vivipara and Campeloma. Nu- 

 merous forms of land shells also occur. 2 



It also seems possible, if not probable, that the present Lymnaeid 

 fauna of this region is a direct descendant from these early times. It 

 seems not at all improbable that the smaller Lymnseas of the United 

 States (techella, desidiosa, etc.) originated here, as well, also, as some 

 of the larger species. That the recent Lymnaa utahensis of Call is a 

 lineal descendant of Meek's Polyrhytis kingi seems evident. Lymncea 

 stearnsi is also an early form of Lymnaa stagnalis. 



It also seems apparent that the Lymnseas of American origin 

 spread from this region, following the river systems as they were 

 formed, and finally mingled with the Asiatic emigrants which reached 

 the plain area by the way of the lowlands of Alaska and British 

 America. This combined fauna worked its way to the higher regions 

 of the Rocky Mountains, where it is found today intermingled with 

 the remnants of the original American stock. It is not believed by 

 the writer that the supposed land connection with Europe via Green- 

 land contributed to any extent in the formation of the present Lymnaeid 

 fauna. 



In the following pages an attempt has been made to bring together 

 all references to extinct American Lymnaeas, together with the original 

 descriptions and figures. No attempt has been made to critically revise 

 the species, but remarks are introduced wherever an opportunity has 



1 Chamberlin and Salisbury, op. cit., Ill, p. 204, etc. 



2 See the interesting paper by Dr. Stearns on the Colorado Desert, which 

 doubtless once formed a part of the Great Basin system. Proc. U. S. Nat. Mus., 



